Industrial wireless network with message authentication

ABSTRACT

A process control and monitoring method for wirelessly communicating with multiple devices is disclosed. The method includes a basestation that communicates with multiple field units via a wireless messages and in accordance with a wireless protocol and multiple field units that receive wireless messages in accordance with the wireless protocol from the basestation and respond to requests from the basestation via wireless messages in accordance with the wireless protocol. The wireless protocol includes messages containing unique identifiers for confirming the authenticity of the wireless messages.

This application is a continuation-in-part of copending, commonlyassigned U.S. patent application Ser. No. 10/449,455, filed May 30,2003, entitled “Non-Interfering Multipath Communications Systems,” theteachings of which are incorporated herein by reference. Thisapplication is related to copending, commonly assigned U.S. patentapplication Ser. No. ______, (Express Mail Label No. EV 324 849 466 US)entitled “Industrial Wireless Network,” filed this same day herewith,the teachings of which are incorporated herein by reference. The presentinvention relates to methods and apparatus for wireless communications,and in particular to, systems for wireless communications among multipledevices for process control, e.g., for monitoring and controllingmanufacturing, industrial, environmental, and other processes.

BACKGROUND OF THE INVENTION

Modern manufacturing techniques often rely on automated monitoring andcontrol systems to assure safe and efficient operation. Such systems useremote sensors and actuators to measure and set equipment states pointsthroughout a process. For example, remote sensors can be positioned tocollect temperature and pressure data and to send that information to acontroller that monitors the overall process. Furthermore, thecontroller can send commands to valves and other actuators to adjustsystem parameters and, thereby, assure optimal system performance.

Electronic monitoring and control via remote sensors and actuators hasproven an effective tool in automating and managing processes, evenprocesses spread over large physical areas. Unfortunately, conventionalcontrol systems are expensive to set-up and maintain. The expense ofwiring communication and electrical lines between remote monitoringunits and central controllers can offset many of the systems'advantages. In addition, the harsh environment found in manufacturingplants, combined with circuitous runs of wires along inaccessibleroutes, can make maintenance difficult.

In addition, such systems are difficult and expensive to change once inplace. As a result, there is a disincentive to improving the process andupgrading the sensors, actuators, and other control equipment. Controlsystems are thus rendered obsolete, costing millions in lostopportunity.

Therefore, a need exists for a flexible, low cost, methods and apparatusfor process control applicable in manufacturing, environmental, andother process control systems.

SUMMARY OF THE INVENTION

The present invention provides apparatus and methods for wirelesscommunication between multiple devices of a process control system. Inone aspect, a basestation and multiple field units communicate via awireless protocol made up of frames containing multiple time slots. In afirst time slot, the basestation sends a wireless start-frame message tothe multiple field units via a wireless signal. The start-frame messagepreferably designates a time slot within a frame for each of at leastone selected field units to respond. The field units respond to thebasestation during the respective time slots with a message containingan identifier. The basestation confirms the authenticity of theresponses by comparing the identifier in the field unit's response witha stored identifier associated with the field unit designated to therespective time slot and accepts the data in the field unit's wirelessmessage only if authenticity is confirmed.

The identifier in the field unit's message provides system security andreduces the chance of passing along bad data. In one embodiment, thefield units send the identifier at a prearranged time within the timeslot and the step of confirming the authenticity further includes thebasestation determining if the identifier was sent at the correct time.In another embodiment, the field units send the identifier at aprearranged frequency and the step of confirming the authenticityfurther includes determining if the identifier was sent at the correctfrequency. In yet another embodiment, the basestation range checks thedata contained in the field unit's message and the step of confirmingthe authenticity further includes the basestation determining if thedata falls within an acceptable range. The basestation accepts the dataonly if the data falls within the acceptable range.

In an additional aspect of the invention, the basestation can alert afield unit when the basestation cannot confirm the authenticity of amessage. After receiving notice of a failure to authenticate, the fieldunit can resend the message. The basestation can also send an alert to asystem user to inform the user of a potential communication problem.

In another aspect of the invention, the basestation utilizes the initialtime slot for a start-frame message which alerts the field units thatone or more of the remaining time slots within the frame are availablefor logon requests from the field units. A field unit confirm theauthenticity of the wireless start-frame message, and if authenticity isconfirmed, the field unit sends a wireless logon request message to thebasestation. The basestation then confirms the authenticity of thewireless logon request message. The aforementioned and other aspects ofthe invention are evident in the drawings and in the text that follows.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be more fully understood from the following detaileddescription taken in conjunction with the accompanying drawings, inwhich:

FIG. 1 is an illustration of one embodiment of the system of the presentinvention;

FIG. 2 is an illustration of synchronous frame in the wireless protocolof the present invention;

FIG. 3 is an illustration of an asynchronous frame of the wirelessprotocol of the present invention;

FIG. 4 is another embodiment of the synchronous frame of the presentinvention;

FIG. 5 is another embodiment of the asynchronous frame of the presentinvention;

FIG. 6 is yet another embodiment of the asynchronous frame of thepresent invention;

FIG. 7 illustrates an additional embodiment of the asynchronous frame ofthe present invention;

FIG. 8 illustrates a multiframe in the wireless protocol of the presentinvention;

FIG. 9 is another embodiment of the multiframe of the present invention;

FIG. 10 illustrates a superframe in the wireless protocol of the presentinvention;

FIG. 11 is a chart of preferred frame lengths and frame durations forvarious baud rates in the wireless protocol of the present invention;

FIG. 12 is a chart of maximum frame transmit duty cycle for various baudrates;

FIG. 13 is a chart of the maximum synchronous times slots during a tensecond period in one embodiment of the wireless protocol of the presentinvention;

FIG. 14 illustrates an exemplary code generator for use with thewireless protocol of the present invention;

FIG. 15 illustrates the structure of a message in one embodiment of thewireless protocol of the present invention;

FIG. 16 illustrates the header block of the message in FIG. 15;

FIG. 17 illustrates a preferred data byte alignment of the message inFIG. 15;

FIG. 18 illustrates a start-frame message of the wireless protocol ofthe present invention;

FIG. 19 illustrates the header block of the start-frame message in FIG.18;

FIG. 20 illustrates a synchronous embodiment of the data block of thestart-frame message in FIG. 18;

FIG. 21 illustrates an asynchronous embodiment of the data block of thestart-frame message in FIG. 18;

FIG. 22 illustrates the error detection and correction block of thestart-frame message in FIG. 18;

FIG. 23 illustrates a synchronous frame field unit data message of thewireless protocol of the present invention;

FIG. 24 illustrates the header block of the synchronous frame field unitdata message in FIG. 23;

FIG. 25 illustrates the data block of the synchronous frame field unitdata message in FIG. 23;

FIG. 26 illustrates the packing of the data block in FIG. 25;

FIG. 27 illustrates the error detection and correction block of thesynchronous frame field unit data message in FIG. 23;

FIG. 28 illustrates an asynchronous frame basestation data message ofthe wireless protocol of the present invention;

FIG. 29 illustrates the header block of the asynchronous framebasestation data message shown in FIG. 28;

FIG. 30 illustrates the data block of the asynchronous frame basestationdata message shown in FIG. 28;

FIG. 31 illustrates the packing of the data block in FIG. 30;

FIG. 32 illustrates the error block of the asynchronous framebasestation data message shown in FIG. 28;

FIG. 33 illustrates an asynchronous frame field unit data message of thewireless protocol of the present invention;

FIG. 34 illustrates a message stream of the wireless protocol of thepresent invention using redundant basestations;

FIG. 35 illustrates a message stream of the wireless protocol of thepresent invention one basestation;

FIG. 36 illustrates the message stream of FIG. 35 after a newbasestation logs on to the wireless network;

FIG. 37 illustrates a message stream of the wireless protocol of thepresent invention after one of the multiple basestations becomesinoperable; and

FIG. 38 illustrates a schematic of multiple basestations connected witha data collection.

DETAILED DESCRIPTION OF THE INVENTION

The present invention includes various embodiments of process controlmethods and apparatus. In one embodiment, a system includes abasestation that communicates with multiple field units via a wirelesssignal and in accordance with a wireless protocol. FIG. 1 illustratesthe application of such a system to a manufacturing process, including abasestation 10 and multiple field units 12. The field units 12 arepositioned at points within the process for wirelessly monitoring and/orcontrolling the system at the direction of basestation 10. For example,the field units can control valves 14, pumps 16, and/or other processequipment. In addition, field units can monitor temperature, pressure,flow rate, fill levels, and other process variables at positions, suchas in fluid conduits 18 and/or tanks 20.

Basestation 10 comprises a convention controller of the type known inthe art, e.g., including a processor, memory, storage, and input/outputcontrol sections. The basestation can be embodied in an embedded system,personal computer, workstation, mainframe, or the like, as known in theart. And, it can be coupled to a user interface and/or communicationsinterface (e.g., for networking) to provide information about systemparameters and/or receive inputs for system control. Basestation 10 alsoincludes a transceiver 24 capable of sending and receiving wirelesssignals in accordance with the wireless protocol discussed in detailbelow. The illustrated transceiver operates at 900 Mhz, although it canoperate at other rates as well, e.g., 2.4 to 5.6 Ghz, and can exercisethe protocol detailed below on top of industry standards and/orproprietary low-level protocols.

Field unit 12 comprises a sensor and/or actuator of the type commonlyknown in the art, as well as, logic for executing commands received fromthe basestation for monitoring and controlling a process, all in theconventional manner known in the art. The field unit 12 can be a so call“smart field device” of the type commercially available in the art, orit can be a conventional field device equipped with a conventionalinterface for use in process control. The field unit includes aprocessor for performing the various task described below, such as, forexample receiving, storing, processing, creating, and/or sendingmessages in accordance with the wireless protocol; collecting,processing; storing, and/or receiving system data; and/or controllingsystem actuators. The field units preferably also include a wirelesstransceiver for communicating with the basestation or other field units.

One skilled in the art will appreciate that, while the basestationillustrated in FIG. 1 is a controller and the field units are fielddevices, those roles could be reversed. Thus, for example, one of thefield devices (presumably a smart field device) could serve as thebasestation and the controller could serve as a field unit. Moreover, itwill be appreciated that other equipment, regardless of whether it is acontroller or field device, could serve as a field unit.

Unlike conventional systems that require long runs of wire betweenremote units and a central unit, the present invention provides awireless network of field units and a basestation for monitoring and/orcontrolling a process. The wireless protocol provides reliable datatransfer having update rates capable of keeping pace with the changingprocess control and monitoring demands of an intricate manufacturingsystem. The result is a flexible, robust system which provides optimizedprocess control without the expense and maintenance problems associatedwith wires.

The basestation and field units communicate via a wireless protocolcomprising frames that define organized segments of communication.Frames can vary in bit length, but are preferably always the samelength. In one embodiment, every frame is 1704 bits in total length. Theframes are divided into time slots in which field units or thebasestation can send or receive a message. Preferably, every frame isdivided into eight time slots.

In one aspect of the invention, the basestation and field unitscommunicate with synchronous and asynchronous frames. Synchronous framesare primarily designed for transmtting measurement data to thebasestation and include a start frame message from the basestation thatassigns the remaining time slots to specific field units. Asynchronousframes are designed for moving large amounts of data between thebasestation and a field unit(s). Unlike synchronous frames, asynchronousframe can include unassigned time slots or time slots assigned generallyto a group (i.e., the field units). For example, a field unit can use anunassigned time slots in an asynchronous frame to log onto the wirelessnetwork.

Although the following description includes specific bit lengths, oneskilled in the art will appreciate that these numbers are exemplary andthe bit lengths can be varied to suit the demands of the system.

Both synchronous and asynchronous frames start with the basestationtransmitting a start-frame message in the first time slot. FIGS. 2 and 3illustrate an exemplary synchronous frame 30 and asynchronous frame 32,respectively, of 1704 bits with a first time slot assigned to thebasestation and used for a start-frame message.

In synchronous frames, the start frame message from the basestationallocates the remaining seven time slots of the respective frame to thefield units. Conversely, allocation of the seven remaining time slots inan asynchronous frame is variable, with one to six time slots availablefor the basestation and one to five time slots available to the fieldunits. The basestation assigns the time slots to the field units in theasynchronous frame depending on how many timeslots have been used by thebasestation and the type of data being transmitted.

In synchronous frames, messages sent by the field units and thebasestation preferably fit within the time slot to which they areassigned. When a field unit needs to transmit more data than can becontained within a single message, the basestation assigns the fieldunit multiple time slots for multiple messages. In some cases, thesetime slots are in adjacent frames. For example, FIG. 4 illustrates onefield unit assigned to time slots seven and eight of a first synchronousframe and to time slot two in the next synchronous frame. Inasynchronous frames, the length of messages are not designed to matchthe length of a single time slot and instead vary depending on how muchdata is being transmitted and the available space within the frame.

FIGS. 5 and 6 illustrate asynchronous frames containing messages ofvarying size. In FIG. 5, the basestation uses the first time slot forthe start frame message plus an additional time slot to request data,while the field unit employs time slots four and five to send therequested data. In FIG. 6, the field unit uses the bulk of the frame tosend a message to the basestation and the basestation uses only thefirst time slot for the start frame message and time slot seven to replyto the field unit's message. Time slot eight is reserved for quiet time.

The basestation preferably reserves the last time slot in everyasynchronous message for quiet time so that the basestation can switchmodes and prepare data for the next message frame. Quiet time ispreferably also included after any message from the basestation. Forexample, 104 bits of quiet time are reserved at the end of anybasestation message, such as, for example at the end of a start framemessage. Quiet time allows the basestation and field units to performfunctions such as switching from transmit to receive mode and changingconfiguration registers. Similarly, quiet time at the end of otherbasestation messages provides time for the basestation radio frequencytransceiver to switch from transmit to receive mode and for the fieldunits to process received data and prepare an ACK/NAK response message.The end of field unit messages preferably also includes quiet time. Forexample, forty bits of quiet time can be reserved between field unitmessages (FIG. 2).

As stated above, field units use asynchronous frames to log into theradio frequency network. Since the basestation and field units useasynchronous frames for multiple purposes, not every asynchronous framewill be available for a login request and field units check thestart-frame message to verify that the asynchronous frame is availablefor login requests.

Asynchronous frames available for a login request preferably have threetime slots reserved for field units to send a login request and two timeslots reserved for the basestation to respond to all login requests. Theremaining two slots are left as quiet time to allow thebasestation/field units time to process the data with the frame and toswitch from transmit to receive mode. Since the basestation does notknow when a field unit may attempt to log into the network, the threefield-unit-login time slots in each login frame are used on a first comebasis and collisions may occur. To minimize the possibilities ofconflicts, field units randomly pick one of the three time slots, aswell as, the asynchronous frame in which to send the login requestmessage.

FIG. 7 illustrates an exemplary asynchronous message with time slotsavailable for login requests by the field units. If the basestationaccepts the field unit's login request, the basestation will preferablyrespond by transmitting information to the field unit concerning theconfiguration of the network, such as, for example the unit's radiofrequency identification number and the location of a future time slotreserved for the device.

The frames of the present invention are preferably grouped intomultiframes having between about two and sixty-three frames andincluding at least one asynchronous and one synchronous frame. Thebasestation conveys the number of asynchronous and synchronous framesper multiframe to a field unit when it logs into a network along withadditional information relating to the network configuration. FIG. 8shows an exemplary multiframe.

The at least one asynchronous frame in the multiframe provides anopportunity for tasks such as logging on and/or sending/retrievingconfiguration information from a device. Since the number of frames in amultiframe is configurable, the cycle can be shortened for smallernetworks to increase the field unit update rate and minimize the timerequired to send/receive asynchronous data messages. FIG. 9 illustratesa shortened multiframe cycle.

The wireless protocol also includes superframes which contain a group ofmultiframes and define the total number of time slots available in anetwork. The size of a superframe is preferably between about one andsixty-three multiframes. FIG. 10 illustrates the organization of anexemplary superframe. The superframe size and the number of synchronousframes per multiframe determine the total number of synchronous timeslots in a network. This number can be calculated base on the number ofsynchronous frames, the number of available time slots in a synchronousframe, and the superframe size.

The wireless protocol can operate at a number of different data baudrates depending upon the application's requirements. Installations witha large number of devices in a small coverage area can preferably be runat a higher data rate while a network containing devices installed in alarge area, especially if the area contains obstructions, can be run atlower data rate to maximize the radio frequency sensitivity.

Regardless of the baud rate, the wireless protocol allows the key timingrequirements to remain the same and the transmit duty cycle to remainunder 10 percent. FIG. 11 illustrates the preferred frame length andtime slot duration at various baud rates and FIG. 12 shows the resultingmaximum frame transmit duty cycle per device for the various baud rates.As described herein, the wireless protocol is optimized for 76800 baud.Although, the protocol works at other baud rates, it will be slightlyless efficient due to the fixed number of time slots per frame and theunnecessarily long inter-time slot quiet times. FIG. 13 shows a chart ofthe maximum synchronous time slots in a 10 second period for differentbaud rates assuming there are forty-nine synchronous frames for amultiframe consisting of fifty frames in total length.

One of the advantages of the wireless protocol is the ability to usetransmissions in the 900 MHz spectrum. Since this spectrum is designatedfor open use, setting up the wireless network will not require speciallicensing.

Preferably, the basestation and field units transmit wireless messagesat a frequency in the range of about 902 MHz to 928 MHz. In yet anotherembodiment, the transmitting frequency changes after each frame. Bychanging frequencies or “hopping” between frequencies, the chance ofnoise creating an interfering signal is reduced. In addition, hoppingfrequencies adds a measure of security because outside systems do notknow which channel will be selected for the next frame.

Frequency hops preferably occur at the end of a message frame after alldata from the field units and/or basestation has been transmitted. A16-stage Gold code sequence pseudo noise generator preferably generatesthe hopping sequence using the lower 16-bits of a unique 32-bit number(MAC address) assigned to the base station as the seed for the lowerlinear feed-back shift registers used in the code generator. As anadditional advantage, Gold code generators produce an equal number of1's and 0's, and will output each possible code only once before thesequence repeats. FIG. 14 shows an exemplary Gold code generator for usein the network of the present invention. A person skilled in the artwill appreciate that other generators can generate the hopping sequence,and particularly those generators capable of producing a large number ofdifferent sequences, each with a low correlation to the other codes.

To minimize processing on the battery power field units, the basestationand/or field units will preferably generate the hopping sequences onceand store them in a table instead of being calculated on the fly.Pre-generated tables and a sequence clock transmitted with basestationmessages can also minimize the time required for field units tosynchronize with the basestation hopping sequence.

All messages in the wireless protocol preferably have the samestructure, including a header block, a data block, and an errordetection and correction block. FIG. 15 illustrates the structure of anexemplary message. The header block of a wireless message preferablyincludes a synchronization preamble, a MAC Address, and possibleadditional bits as shown in FIG. 16. The first 32 bits of a header blockpreferably consist of an initial alternating 10101010 . . . preambleused by the receiving device to synchronize with a transmitted radiofrequency data stream. The next 32 bits of the header block arepreferably occupied by the MAC address field, which is used in thepattern match registers of the receiving radio frequency transceivers.The MAC address can include 4 transition bits, one bit to indicate thetype of message being sent, and a 27-bit unique number assigned everyradio frequency device used with the network. A message type bit of 1indicates a frame start or a synchronous frame data message and amessage type bit of 0 indicates an in-frame asynchronous data message.

The data block portion of a message preferably contains the actualpayload of a packet, with the size of the data block varying based onthe type of frame and the information contained therein. All data ispreferably sent out with the most significant bit first. An exemplarybyte alignment is shown in FIG. 17, including at least one 0 to 1 or 1to 0 bit transition for every eight bits transmitted and the use of 8bit groups.

The final block in the message, the error detection and correctionblock, preferably contains a number of bits used to determine if themessage has any errors, as well as, to correct a limited number of biterrors.

Start-frame messages, like all other messages, preferably contain threeblocks as shown in FIG. 18. The first block, the header block, isillustrated in FIG. 19, and preferably includes the 32 bit alternating10101 . . . preamble, followed by the basestation's 32 bit MAC address.The message type is set to a value of 1 to distinguish the message as astart-frame message. The header also holds the hopping sequence clock,which the listening devices use to synchronize with the network'sfrequency hopping sequence, and information on the current multiframenumber, the current frame number and type, and the total frames permultiframe.

The structure of the data block in a start frame message depends onwhether the frame is synchronous (FIG. 20) or asynchronous (FIG. 21). Insynchronous start-frame messages, the data block consists of 3 fields,each of can contain information for a basestation or field unit that mayhave been allocated a time slot within the frame. All fields in the datablock are preferably bit field variables with the most significant databit holding data from the field unit that has been allocated time slottwo and the least significant data bit for holding data for the fieldunit with time slot eight. The basestation uses the first field, theAsynchronous COM Request Field (FIG. 20), to indicate to one or morefield units that there is data waiting to be transferred to the fieldunit in one or more of the future asynchronous message frames. A bitvalue of 1 indicates that the field unit should start listening to allasynchronous message frames for possible data. The basestation uses thesecond field, the Time Slot Acknowledge Field, to ACK/NAK a message sentpreviously in the same synchronous multiframe/frame. If the messagereceived by the basestation contained errors or was never received, thebit value will be 0. If the basestation successfully received themessage, the bit value will be 1. The basestation can use the thirdfield, the Requested Measurement/Information Channel Field, to overridethe selection of data normally set by a field unit in a time slot and torequest specific measurement data.

In asynchronous start-frame messages, the data block in the start framemessage contains different information. As shown in FIG. 21, the firstfield in the data block contains the radio frequency identification ofthe field unit for which the asynchronous frame is reserved. Thebasestation sets this value to 0 if the frame is not reserved and anyfield unit may attempt to send a login request message. The other fieldin the data block contains the length of the asynchronous data blocksent from the basestation to the field unit. If the basestation sends nodata in the remainder of the frame's time slots, the basestation setsthis value to 0.

The final portion of the basestation's start-frame messages includes anerror detection and correction block. FIG. 22 illustrates an exemplaryerror block having a 16-bit CRC and eight error correction bits. AHamming code can provide a basic level of error correction with only asmall overhead of a few added bits and a short processing time toencode/decode the message. The error detection and correction blockprotects all bits in the message frame except for the synchronizedpreamble and MAC address fields in the header block.

In response to a basestation's synchronous start-frame message, a fieldunit preferably replies with a synchronous frame data message. Withreference to FIGS. 23 through 27, an exemplary synchronous frame datamessage includes a header block, a data block, and an error detectionand correction block. The header block, shown in FIG. 24, starts withthe usual 32 bits of alternating 1010 . . . for a preamble, followed bythe field unit's MAC address. The field unit sets the message type bitto a value of 1 to distinguish this message as a synchronous data type.The data block, shown in FIG. 25, contains seven bytes of data with anadditional byte's worth of transition bits. To optimize the packing ofthe field unit's information, the field unit produces synchronousmessages having data bytes packed in groups of 8 bytes with the mostsignificant bit of each of the seven data bytes being stripped off andstored in the lower seven bits of the last byte. The field unit theninserts a transition bit in the most significant bit of the all theeight bytes. The preferred data packing structure is illustrated in FIG.26.

The field unit can assign data bytes to deliver specific information.For example in one assignment scheme, the first byte delivers the statusof the field unit (8 bit flags); the second through fifth byte deliverthe value for the measurement/information channel being delivered (thismay be anything from a floating point value to 4 individual bytes and isdefined by the valued of later bytes); the sixth byte delivers the fieldunit type; and the seventh byte delivers measurement/information channelinformation.

Finally, as shown in FIG. 27, the field unit data message can include anerror detection and correction block identical to the error block usedin the basestation start-frame message.

During asynchronous frame, the basestation can send two different typesof messages, an asynchronous start-frame message and an asynchronousdata message. An exemplary asynchronous data message is illustrated inFIG. 28 and includes a header block, a data block of variable size, andan error detection and correction block.

An asynchronous data message from a basestation starts with a shortheader block (FIG. 29), including a 32 bit preamble, a 32-bit MACaddress, a command/data type field, and a data block length field.Unlike other basestation messages, the message type bit in the MACaddress field will be set to 0 to distinguish this message as anin-frame asynchronous data message. Since this field is used to set thevalue of the pattern match registers on radio frequency transceivers,changing this bit will avoid the problem of field units accidentallyreceiving this message when attempting to synchronize with thebasestation start-frame message stream.

The data block in the basestation asynchronous data message can varybetween about 14 and 848 bits. Due to the requirement by the radiofrequency transceivers for transition bits, the actual data that can becontained in this block is about 1 to 91 bytes. FIG. 30 illustrates anexemplary data block. To optimize the packing of the basestation's data,the basestation packs the data bytes in groups with the most significantbit of each data byte stripped off and stored in the lower seven bits ofthe last byte in the group. If the message consists of less than sevendata bytes or of a number of bytes not a multiple of seven, the lastbyte in the group will hold the most significant bits of the previousbytes. FIG. 31 illustrates basestation data packing.

The final portion of a basestation asynchronous data message contains anerror correction and detection block as shown in FIG. 32. This block issimilar to the error detection blocks used elsewhere in other messagesand preferably includes a 16-bit CRC, as well as, Hamming code errorcorrection bits. In one aspect, the basestation asynchronous datamessage error detection block preferably includes twelve errorcorrection bits, instead of the normal 8 bits, to handle the additionallength of some asynchronous data messages.

Field units can also send asynchronous data messages, which areidentical to the basestation asynchronous data messages except for thesubstitution of the field unit's MAC address in the header block. FIG.33 illustrates a field unit asynchronous data message.

In one embodiment, the basestation and field units encrypt the messagestransmitted in the wireless protocol. For example, the system caninclude a 48-bit weak encryption scheme to encrypt all messages sent byeither a basestation or a field unit.

The simplified set-up of the wireless network reduces user errors andspeeds instillation. A user only needs to input the radio frequency baudrate, the MAC address of the primary basestation on the network, the MACaddress of the all field units on the network, and/or an encryption keyinto the various network devices. Set-up preferably begins with a sitesurvey to determine a good physical location for the basestation. A userthen mounts the basestation, and selects the baud rate through either akeypad attached to the basestation or a PC configuration toolcommunicating with the basestation over a secure wired interface (i.e.,RS-485 serial or Ethernet cable). MAC addresses of all the field unitsare also preferably entered into the basestation.

After the basestation has been installed and is operating, the user canconfigure the field units with the baud rate, the primary basestation'sMAC address, and the network encryption key (if used). All values arepreferably entered into the field unit using a wired connection. Afterconfiguration, the field units can then log into the network. When thefield unit's login request is accepted by the basestation, thebasestation will preferably send the field unit any other neededinformation. For example, the basestation can send the field unit theradio frequency identification, the value of various network parameters(e.g., the number of asynchronous frames per multiframe), and thelocation of a future synchronous time slots that has been reserved forthe device.

A number of different elements, taken together, provide security for thenetwork and result in secure data transmission. For example, frequencyhopping provides a basic level of privacy because the shear number ofdifferent hopping sequences makes it unlikely that neighboring networkswould have the same hopping sequence or could easily decipher thepattern.

The use of unique identifiers associated the basestation and field unitsfurther protect the system by providing a method for authenticatingmessages. For example, each message sent by a network device can containa unique identifier which the receiving device uses to confirm theauthenticity or origin of the received message. In one embodiment, theidentifier is a MAC address. The receiving device can check the MACaddress against a stored list of MAC addresses associated with thedevices on the network. If the received MAC address does not match a MACaddress on the list, the receiving device preferably does not accept themessage. In addition, where a device is assigned to a specific timeslot, any message received during that time slot can be checked againstthe stored MAC address for the device assigned to that time slot. If theMAC address does not match, the receiving device preferably rejects themessage.

The basestation and field units can also use the basic timing of themessaging protocol to authenticate the messages. In one embodiment, thenetwork devices check the time at which the identifier is received toauthenticate a message. In another embodiment, the network devices usethe frequency at which the MAC address is received to authenticate amessage. In addition, the timing and frequency of the message as a wholecan be used in the authentication process. The basestation or the fieldunits can then reject any message not sent at the correct time or at thecorrect frequency. As an additional authentication measure, devices onthe network can range check data to determine if the data falls within avalid range. If data falls outside the measurable range of a sensor oris not a physically possible result, the receiving device does notaccept the message and/or the data contained therein.

As an additional authentication measure, devices on the network canrange check data to determine if the data falls within a valid range. Ifdata falls outside the measurable range of a sensor or is not aphysically possible result, the receiving device does not accept themessage and/or the data contained therein.

Authenticating messages and/or data protects the system from passing onrandom or garbage information. For example, if signal interferencegarbles transmitted data, the authentication scheme minimizes the chanceof passing on invalid data. Instead, the sending device will note theerror and the data can be resent. Alternatively, or in addition, thebasestation can generate an error message for the system's user.

Redundant basestations can further improve network reliability. Forexample, multiple basestations operating on the same hopping frequencyprovide multiple paths for data receipt. In one embodiment, basestationscan take turns on a round-robin basis with the job of network masterpassing between different basestations. The master basestation cantransmit the start frame message for an entire superframe to synchronizethe network. All other sub basestations will listen for the messagestream from the current master to determine where the network is in thecycle. The next basestation to assume the role of master (secondarymaster) will adopt the MAC address of the last master (primary master)basestation and use the hopping sequence from the primary master. FIG.34 illustrates one exemplary embodiment of the redundant basestationmessage stream.

The secondary masters preferably log onto the network using the sametechnique as the field units and are assigned positions to handle withinthe message stream. Until secondary masters join the network, theprimary master handles the entire message stream. FIG. 35 shows amessage stream with only one basestation. As more basestations log ontothe system, the primary master assigns the secondary basestationspositions within the message stream (FIG. 36). If later a basestationbecomes inoperable and its messages can no longer be heard by otherbasestations on the network, the previous basestation can take itsposition. FIG. 37 illustrates a message stream where basestation twobecomes inoperable and basestation one assumes its duties.

In systems containing multiple basestations, the data from eachbasestation is preferably collected by a data collector/concentrationelement. In one embodiment, the data collector is a PC or an embeddeddevice. FIG. 38 shows a schematic of multiple basestations connectedwith a data collector.

A further understanding of the invention may be attained by reference tocopending, commonly assigned U.S. patent application Ser. No. ______ ,(Express Mail Label No. EV 324 849 466 US) entitled “Industrial WirelessNetwork,” filed this same day herewith, the teachings of which areincorporated herein by reference. A further understanding of oneembodiment of the invention may be attained by reference toaforementioned incorporated-by-reference U.S. patent application Ser.No. 10/449,455, filed May 30, 2003, entitled “Non-Interfering MultipathCommunications Systems.” One skilled in the art will appreciate furtherfeatures and advantages of the invention based on the above-describedembodiments. Accordingly, the invention is not to be limited by what hasbeen particularly shown and described, except as indicated by theappended claims. All publications and references cited herein areexpressly incorporated herein by reference in their entirety.

1. A process control and monitoring method for wirelessly communicatingwith multiple devices, the method comprising: a. sending a wirelessstart-frame message from a basestation to multiple field units in afirst time slot of a frame, the start-frame designating a respectivetime slot within the frame for each of at least one selected field unitsto respond; c. sending, with the selected field units, during therespective time slot designated for that field unit, a response to arequest in the basestation's wireless start-frame message, the responsecontaining an identifier; d. confirming, with the basestation, theauthenticity of the responses received during the respective time slotsfrom the selected field units by comparing an identifier in thatresponse with a stored identifier associated with the field unitdesignated to that respective time slot, and e. accepting data in thefield unit's wireless message only if authenticity is confirmed.
 2. Themethod of claim 1, wherein the selected field units send the identifierto the basestation at a prearranged time within the time slot, and thestep of confirming the authenticity further includes determining if theidentifier was sent at the correct time.
 3. The method of claim 1,wherein the selected field units send the identifier to the basestationat a prearranged frequency, and the step of confirming the authenticityfurther includes determining if identifier was sent at the correctfrequency.
 4. The method of claim 1, wherein the basestation alerts afield unit when a response is not accepted.
 5. The method of claim 1,wherein the start-frame message and the response are encrypted.
 6. Themethod of claim 1, wherein the identifier is a unique numeric code. 7.The method of claim 1, wherein data communicated in the response isrange checked and the response is not accepted if any data is not withina valid range.
 8. The method of claim 1, wherein the start-frame messagefrom the basestation includes an identifier and the selected field unitswhich receive the start-frame message from the basestation authenticatethe messages using the identifier.
 9. A process control and monitoringmethod for wirelessly communicating with multiple devices, the methodcomprising: a. sending a wireless start-frame message from a basestationto multiple field units in a first time slot of a frame, the wirelessstart-frame message designating at least one time slot in the frame forreceiving wireless logon request messages from the field units; c.confirming, with at least one field unit, the authenticity of thewireless start-frame message; d. sending, with the at least one fieldunit, a wireless logon request message from the field unit to thebasestation if authenticity is confirmed by that at least one fieldunit; and e. confirming, with the basestation, the authenticity of thewireless logon request message sent from the at least one field unit.10. The method of claim 9, wherein confirming the authenticity includescomparing an identifier within the received signal with storedidentifiers.
 11. The method of claim 9, wherein confirming theauthenticity includes comparing the time at which a message is receivedwith a prearranged time for sending the message.
 12. The method of claim9, wherein confirming the authenticity includes comparing the frequencyat which a message is received with a prearranged frequency.
 13. Themethod of claim 9, wherein confirming the authenticity includescomparing received data with the expected range for the data, andrejecting any data not within a valid range.
 14. The method of claim 9,wherein the basestation does not respond to the wireless logon requestmessage if the logon message fails authentication.
 15. The method ofclaim 9, wherein the field unit does not respond to the basestation ifthe wireless start-frame message fails authentication.